In the previous chapter, we discussed radiation patterns. To better analyze antenna radiation, a reference point is required. The radiation of an isotropic antenna fulfills this role.
Definition
Isotropic radiation refers to radiation from a point source that propagates uniformly in all directions, with the same intensity regardless of the measurement direction. The improvement of an antenna's radiation pattern is always evaluated using isotropic radiation as a reference. If the radiation is equal in all directions, it is called isotropic radiation.
•A point source is an example of an isotropic radiator. However, such isotropic radiation is practically impossible to achieve, because any antenna radiates its energy with some directivity.
•Isotropic radiation is essentially the same as omnidirectional radiation.
•When viewed in 3D, it has a doughnut-shaped pattern; when viewed in 2D, it has a figure-eight pattern.
The figure above shows the radiation pattern of an isotropic (or omnidirectional) radiator. Figure 1 presents the doughnut-shaped pattern in 3D, while Figure 2 shows the figure-eight pattern in 2D.
Gain
An isotropic radiator has unity gain, meaning the gain factor is 1 in all directions. When expressed in decibels (dB), this is referred to as 0 dB gain (zero loss).
Equivalent Isotropically Radiated Power (EIRP)
According to the standard definition, the amount of power radiated by an isotropic antenna to produce the peak power density observed in the direction of maximum antenna gain is called Equivalent Isotropically Radiated Power.
When the radiated energy of an antenna is concentrated on one side or in a specific direction, and the radiation in that direction is equivalent to the isotropic radiated power of the antenna, this radiation is referred to as EIRP.
Gain
Although isotropic radiation is only an idealization, it represents the best possible radiation an antenna could achieve. Such an antenna has a gain of 3 dBi, where 3 dB corresponds to a factor of 2, and “i” denotes the multiple relative to the isotropic condition.
If the radiation is focused within a certain angle, the EIRP increases with the antenna gain. The gain of an antenna is best achieved by concentrating its radiation in one direction.
Effective Radiated Power (ERP)
If the radiated power is calculated with reference to a half-wave dipole antenna instead of an isotropic antenna, it is referred to as Effective Radiated Power (ERP).
ERP(dBW)=EIRP(dBW)−2.15 dBi
If EIRP is known, ERP can be calculated using the formula given above.
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Post time: Apr-17-2026
